Barstar has been studied by CD and scanning microcalorimetry in extended range pH and salt concentration. It was shown that the protein is stable in very narrow range of pH 6-8 and undergoes conformational transition above and below these pH. At pH below pH 3 barstar exhibits characteristics of denatured proteins classified as a molten globule state. At neutral pH barstar has stable native structure. Temperature increase leads to the disruption of its structure in a co-operative way. This process is accompanied by significant enthalpic effect and an increase in the heat capacity of the protein. The remarkable feature of the barstar is that the enthalpy of disruption of its native structure was found to be much lower in value than for other small globular proteins measured at comparable temperature. From this point of view barstar appear to be close in this thermodynamic characteristic to a-lactalbumin or others globular proteins which are considered to have flexible tertiary structure.